It is a familiar practice in the decorating art to print a multi-color pattern on a substrate surface by transfer (offset) printing of individual ink patterns. Each individual pattern is formed with a different colored decorating ink. The substrate may, for example, be a glass, glass-ceramic, or ceramic body. Individual ink patterns are supplied separately to an elastomeric transfer (offset) roll from a series of patterned surfaces. Each surface is patterned to form an individual ink pattern for delivery to the transfer roll. A patterned surface may be provided by a gravure plate or roll, a lithographic roll , a typographic roll, or a silk screen.
Normally, when printing to non-porous surfaces, the ink pattern dries to a cohesive, tacky state on the elastomeric roll by evaporation or solvent penetration into the roll. The ink pattern is then brought into intimate mechanical contact with the substrate surface. It is completely transferred when it has sufficient cohesion and greater affinity for that surface than it has for the transfer roll. In some cases, when using solvent-based inks, complete transfer is not obtained, and print quality, particularly in regard to definition, may be reduced as a consequence.
Once the transferred ink pattern is sufficiently dry, successive patterns of other colors may be applied, each from its own pad or roll, to create a multi-colored pattern. Thus, each color pattern must be applied separately to form the multi-colored pattern.
U.S. Pat. No. 4,445,432 (Ford et al.) discloses a method and apparatus which utilize a double offset technique for applying thermoplastic decorating inks onto a substrate to form a multi-color pattern. In this procedure, an ink pattern of each color is successively transferred onto a collector roll to form a fully registered, multi-colored ink pattern on the collector roll. This multi-colored pattern is then transferred to the substrate in a single printing step. A primary advantage obtained by this procedure, with respect to conventional offset gravure practice, is that of superior registration, particularly for substrates of complex geometry. The collector roll renders pattern registration independent of substrate geometry.
U.S. Pat. No. 4,549,928 (Blanding et al.) describes using a double offset technique for printing the phosphors and the black matrix on color TV panels. In this procedure, thermoplastic, pressure-sensitive inks, corresponding to the red, green, and blue color phosphors and the black matrix, are applied separately to the collector roll to form the desired multi-color pattern. This pattern is then transferred as a complete pattern to the TV tube panel.
The double offset printing techniques described in the Ford et al. and Blanding et al. patents employ pressure-sensitive, hot-melt inks. These inks are printed from heated gravure rolls. The inks cool sufficiently on the offset surfaces to develop the cohesive strength required to achieve 100% ink transfer between the offset surfaces and the collector roll, and between the collector roll and the substrate.
To obtain complete transfer, the cohesive strength of the ink must exceed the adhesive strength to the surface of the transferring member. Adhesion must, of course, be greater to the receiving surface than to the releasing or transferring surface. This means that inks must exhibit less adhesion to the first offset surface than to the collector, and less adhesion to the collector than to the final substrate. Heretofore, silicones have been used as the materials for both offset surfaces.
The inks generally used have been thermoplastic in nature. They have been printed onto the first offset surface from heated gravure plates or rolls, and, even more recently, by use of heated screens. Once on the offset surface, the inks cool, developing sufficient cohesive strength for transfer. The inks were formulated to retain sufficient tack after cooling so that they would adhere to surfaces simply by applying sufficient pressure. Since the inks were subsequently fired to consolidate the pigmented glass frits and remove the organics, no particular durability of the organics was required. It has been demonstrated, however, that heat reactive (thermoset) inks can be printed as hot melts at 60.degree.-70.degree. C. and cured with a post-cure at higher temperatures of 150.degree.-200.degree. C.
The double offset procedure, employing hot-melt inks, has been found particularly useful for decorating articles such as dinnerware. However, problems have been encountered in attempting to apply the procedure to printing of precision patterns. This is particularly true for surfaces that may subsequently be required to withstand elevated temperatures, for example 250.degree. C. In particular, the pressure-sensitive, hot-melt inks are not stable at elevated temperatures. Temperatures above 150.degree. C. may result in plasticizer volatilization and oxidative degradation of the typical organic ingredients employed. Further, prolonged heating at 250.degree. C. can even result in distortion of an ink pattern. Excessive flow of the ink elements of the pattern may occur on the substrate surface as viscosity decreases with increase in temperature.
It is possible to develop hot-melt inks that can be subsequently cured thermally or by radiation. However, the heated ink procedure is not preferred where precise registration is required. Slight temperature variations, either in the print surface, or through conduction into the printing apparatus, can result in registration variability.
It has been proposed by K. Mizuno and S. Okazaki, in Japanese Journal Of Applied Physics, Vol. 30, No. 118, Nov., 1991, pp. 3313-3317, to produce a color filter by a process wherein ink on a transfer roll is cured by UV exposure, and then transferred to a glass coated with an adhesive layer. It would, of course, be desirable to collect and transfer a complete pattern, and to do so without the need for an adhesive layer.
It has also been proposed to produce a color filter by photolithography in the form of film. The pattern may then be inspected, and, if necessary, discarded without printing. If the pattern is accepted, the film is transferred directly to the glass substrate. This proposal is described by K. Ikiaki in a publication entitled "Low Cost Technology for Producing LCD Color Filters Transfer Print Method" In Nikkei MIr Vo1: 58, pp. 83-87 (90-04). The process still involves photolithography.
In addition to the items mentioned above, attention is also to a publication by W.C. O'Mara, entitled "Active Matrix Liquid Crystal Displays Part I: Manufacturing Process" appearing at pages 65-70 in the Dec. 1991 issue of Solid State Technology.
It is then a basic purpose of the invention to provide an improved method of printing a multi-color ink pattern on a substrate surface. A further purpose is to provide a method of producing a multi-color ink pattern on a substrate surface that is particularly adapted to producing a precision pattern. Another purpose is to provide a method of producing a multi-color pattern that will withstand elevated temperatures up to about 250.degree. C. without needing to first pyrolyze organics, and then to melt a glass frit at an even higher temperature. A further purpose is to adapt the double offset printing technique to the production of precision multi-color ink patterns.
SUMMARY OF THE INVENTION
The invention resides in a method of printing a multi-color ink pattern on a substrate surface which comprises the steps of arranging a series of surfaces in which each surface has a pattern that is unique to one of the colors, and to the pattern of that color, in the multi-color pattern, supplying to each patterned surface a radiation-curable ink formulation having an appropriate colorant to form an ink pattern thereon, transferring individually the colored ink pattern from each patterned surface to a collector roll, forming a composite of the color patterns on the collector roll, increasing the cohesiveness of the ink sufficiently to permit complete transfer of the pattern, and transferring the composite pattern in its entirety to the substrate surface.